This invention is directed to a method and apparatus by which a charged particle beam of electron or ions is focused, spot-shaped and blanked so as to permit high-speed writing of the beam in a desired pattern on a target.
Electron and ion beams are used to expose lithographically sensitive resist material on a semiconductive wafer so that after removal of the non-masking portions of the resist, the wafer can be doped or etched. As beam spot resolution improves, finer lines can be drawn, with closer spacing, but such small spots require considerable time to expose large areas. The writing speed of conventional electron beam lithography systems in which the demagnified image of a thermionic electron source serves as the writing space is inadequate for submicron circuit fabrication purposes.
An alternative approach to submicron electron and ion beam lithography is to use shaped spots. Several investigators have shown that variable shaped spots with good edge resolution can increase the writing speed potential dramatically, as outlined in the following papers: M. G. R. Thomson, R. J. Collier, and D. R. Herriott, Journal of Vacuum Science Technology 15, 891 (1978); and H. C. Pfeiffer Journal of Vacuum Science Technology 15, 887 (1978). In fact, for minimum feature sizes of 1-2 microns, this approach has been proven in an actual production environment (see: G. J. Guiffre, J. F. Marquis, H. C. Pfeiffer, W. Stickel, 15th Symposium on Electron, Ion and Photon Beam Technology, Boston (1979).
As the beam sweeps over the area to be exposed, it is blanked at the boundaries of the area. When the beam is a spot image of the source or is a shaped beam with the blanking done above the shaping aperture, even with fast blanking there is a margin of the exposure area where the exposure is washed out. The exposure is not sharply cut off at the edge of the feature to be exposed, even with fast blanking. To produce sharp exposure boundaries, the shaped writing spot must be moved in steps. The writing spot remains in each area until exposure is achieved and then is shifted to the adjacent area. When the edge of the feature to be exposed is reached, the writing spot provides full exposure to the edge of the feature, and then fast blanking stops the exposure. This method defines the feature to be exposed with fairly sharp edges to the exposure. However, stepping of the spot results in inefficient time utilization and in excessive band width needs for the beam deflection system.
Therefore, there is need for a method and apparatus for spot-shaping and blanking a focused beam which permits sweeping of the beam to minimize exposure time and needed deflection bandwith and yet provides an accurate and sharp edge definition of the feature to be exposed so that exposure can be readily and economically mechanized and programmed to provide rapid exposure.